In a conventional injection moulder, synthetic plastic is heated to a molten state and the molten plastics material is then forced at high pressure, through gates, sprues and runners to a mould cavity.
The rates at which the mould cavities can be filled are limited by the rate at which the molten plastics material can flow through the narrow sprues, runners and gates. The molten plastics material has to be fed to the mould cavity at high pressures, which are costly to apply and which require substantial structural strength of the mould, feed equipment, etc. These disadvantages can be overcome by increasing the cross sections of the passages via which the molten plastics material is fed to the mould, which also allows molten plastics material with inclusions such as long fibres, particulate matter from recycled plastics, or the like, to be used. With increased cross sections of the feed passages, lower feed pressures are required.
However, large cross sectional feeds to mould cavities have the disadvantage that the port where the feed enters the mould, needs to be closed off when the molten plastics material is allowed to freeze in the mould. This can be achieved by accumulating the molten plastics material needed to fill the mould cavity, in a cylindrical holding chamber adjacent the mould and closing the port with a piston, the leading face of which becomes part of the peripheral wall of the mould cavity, when closed. However, in apparatus of this type, the holding chamber is open to the mould cavity and molten plastics material flowing into the holding chamber contacts the part of the mould cavity immediately adjacent the feed port, where it starts to freeze before the piston forces the molten plastics material into the chamber.
Further, the piston face in this type of mould apparatus is typically internally cooled to cool with the rest of the mould wall, when the molten plastics material in the mould cavity is frozen. When the piston is withdrawn to refill the holding chamber for a following mould cycle, the molten plastics material that flows into the holding chamber contacts the cold piston face and begins to freeze. The partial freezing of the molten plastics material in some areas, prior to the piston filling the mould, increases the viscosity of the molten plastics material and thus requires higher feed pressure to fill the mould cavity and is prone to leaving marks on the moulded products.
The disadvantages of filling a large cross sectioned holding chamber adjacent the mould cavity, before urging the molten plastics material into the mould cavity, are ameliorated to some extent in the invention disclosed in U.S. Pat. No. 6,464,910, to Smorgon et al. This patent discloses a moulding cycle in which molten plastics material is fed to an accumulator, from where it is fed to the mould cavity at low pressure, via a large cross sectioned passage and wherein the feed port of the passage leading into the mould cavity is closed by a piston of a valve. The molten plastics material is thus not accumulated in a holding cavity immediately adjacent the mould, where some of the molten plastics material may have frozen.
However, in the Smorgon et al process, molten plastics material is fed continually from an extruder to the accumulator and from the accumulator to the mould cavity, so that there is no volumetric control over the quantity of molten plastics material fed to the mould cavity. Instead, the pressure within the mould cavity is measured. The result is that the mould cavity is filled completely before the piston of the valve starts to close the feed to the mould. The valve has large cross sectional dimensions and the advancement of the valve piston feeds a considerable volume of additional molten plastics material into the mould, thus causing over filling of the mould. Smorgon does not describe what happens to the overflow of molten plastics material fed to the mould, but it is presumably received in an overflow reservoir and goes to waste.
Further, in the Smorgon et al process, the molten plastics material is fed continuously under low pressure from the accumulator to the mould cavity until the mould cavity is full and the pressure in the mould increases. Only when this increase in pressure is detected, does the valve piston start its movement to close the mould. It can thus safely be assumed that the flow of the molten plastics material is momentarily interrupted before the valve piston movement starts. The interruption of the flow momentarily increases the residence time of the molten plastics material in the valve adjacent the mould and causes changes in the rheology of the plastics material. The valve piston then forces this material into the mould cavity, resulting in visible marks and/or local weakness within the product. Apart from the stagnation that occurs in the valve, the melt front velocity of molten plastics material that flows into the mould is also momentarily disrupted, which affects the physical properties and appearance of the moulded product.
One object of the present invention is to provide an improved moulding method and apparatus which allow molten plastics material to be fed uninterruptedly to a mould cavity through a large cross sectioned passage.
Another object of the present invention is to provide an improved moulding method and apparatus which limit wastage by limiting overfilling of a mould cavity with molten plastics material which is fed uninterruptedly to the cavity.